Glide tables technically have a RowID already built in. Adding a RowID column just exposes and makes visible the RowID that was already assigned to the row. That is of course different from Google sheets, where RowID’s are not generated until you add the RowID column.
So in your case, it’s possible that glide didn’t deem it necessary to show the RowID column since it was already there behind the scenes. That’s probably why you got the same ID’s once you added the column to the table.
I would agree though that the column should have been visible from the start.